Abstract
Bilateral radio-frequency (RF) lesions of the substantia nigra retarded Pavlovian eye-blink (EB) conditioning without affecting concurrent heart rate (HR) conditioning. Dopamine (DA) depletion occurred only in the caudate nucleus, whereas norepinephrine (NE) depletion was limited to the hypothalamus. Bilateral 6-hydroxydopamine (6-OHDA) lesions of substantia nigra retarded acquisition of both EB and HR responses. Six-OHDA lesions produced significant NE depletion in the nucleus acumbens/septi, frontal cortices, and hypothalamus, as well as DA depletion in the caudate nucleus. Trials required to reach EB conditioning criterion was significantly correlated with caudate DA levels. The magnitude of conditioned bradycardia was on the other hand significantly correlated with hypothalamic NE levels. These results suggest that interruption of the nigro-striatal dopaminergic pathway retards Pavlovian somatomotor learning without affecting concurrent autonomic learning, although the latter may depend on an intact ascending NE pathway to the hypothalamus, which passes through the tegmentum and thus is also destroyed in some cases by substantia nigra lesions.
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